Many folks seem to prefer speaker level connections, hearing and/or thinking that there is a better match/integration with the main speakers if the subwoofer receives the same signal from the amplifier as the main speakers. I can't say that I've noticed a difference between the high level and speaker-level connections into ACI or REL subwoofers that could not be dealt with by an adjustment of the subwoofer's level control (but all the usual caveats apply).
The ACI offers both connection options, but, due to some peculiarities of my setup, I often detected a low-level hum when I used the speaker level connecters. The hum disappeared when I changed to shielded ICs between my preamp. outputs and the low-level inputs of my Titan II LE.
This low-level connection also allows me to insert, between my preamp. and amp., a pair of ACI's virtually transparent (to me at least) 65 Hz. 12 db./octave high-pass filters which seem to relieve a little congestion in my Vandersteen 1C's bass and lower midrange.
When I run the Vandersteens full-range (unfiltered), I lower the Titan II's crossover points a little - to just a hair above their lowest settings, and the subwoofer fills in the bottom octave Rel-style.
I usually employ the high pass filters, but when there are few high-amplitude bass transients to challenge my 2-way Vandersteens, the Rel method has worked fine.